1. Field of the Invention
The invention relates to a method for storing data on multiple volumes of a media set and, more particularly, to storing data on multiple optical disk volumes in an optical disk autochanger.
2. Related Art
A format describes the manner in which a file is organized on a piece of storage media such as an optical disk. The simplest formats are sequential. In a sequential format, the files must be written to and read from the media in a sequence from beginning to end. Non-sequential formats do not require a specific reading or writing order and allow file and directory information to be modified without having to rewrite the entire data set. Allowing the file and directory information to be modified, however, significantly increases the complexity of the format. Since file structures do not merely physically follow one another as in the sequential format, the non-sequential format requires that the file structures be explicitly linked together. The links are used to locate specific files or specific portions of files. When updating information about a file, these links must be maintained. Maintaining these links requires that some file structures be rewritten.
The ISO/IEC 13346 standard provides a standard file format for optical disk storage. This format is non-sequential in nature, making it suitable for use in on-line storage systems. The ISO/IEC standard format is suitable for use in file sets stored on a single media surface such as a stand alone optical drive. However, when such file formats are used for a file set stored on multiple surfaces, the links between the file structures will inevitably cross surface boundaries. As a result of the links crossing surface boundaries, even very simple operations, such as reading a directory can become a complex operation. This is because whenever the links cross surfaces, updating and following these links require an optical disk swap.
Swap times are approximately three orders of magnitude longer than seeks times. It is possible that the disk swapping required to perform such a simple task as listing the files in a directory may take tens of minutes. Not only is this unacceptable from a performance standpoint, unnecessary swapping results in added stress on the optical disk autochanger. In fact, listing the files in a directory may cause an autochanger to appear as if it is being given a stress test. Thus, swap minimization is a key challenge for file systems on autochangers.
One approach to swap minimization is to disallow any links from crossing surface boundaries. This may be implemented by assigning separate file sets to each surface. This may work well in some situations, but requires extra software or administrative effort to prevent over-full or empty surfaces. In many cases, the administrative effort is awkward and resource-consuming.
Another approach to swap minimization is to allow links to span surfaces, but to keep the linked structures on a single surface or group of surfaces which are always on-line. That is, the file data stored on the optical disk surfaces is separated from the information about the files. The information about the files includes, for example, directory hierarchy information, file names, file attributes, volume free space and volume attributes. This information is known as meta-data.
In this method, the meta-data may be stored, for example, on a non-removable magnetic hard disk which is dedicated to storing the linked structures. Thus, in this method, data is maintained on the optical disk surfaces while the meta-data is separated and placed on a dedicated storage means. When the optical disks are to be removed from the autochanger, however, the meta-data must be written to the optical disks so that a user is presented with a stand-alone media set.
The ISO/IEC standard assures file compatibility from one application to another. However, when the meta-data is written to the optical disks, it is not written in any way which allows it to be quickly read. Thus, when the optical disks are again loaded into an autochanger, many swaps may be required to move the meta-data back to a dedicated magnetic disk.
What is needed is a method for storing the meta-data on a multiple volume media set in a format that is compatible with the ISO/IEC standard so that compatibility is maintained, and which also allows the meta-data to be efficiently read from and written to the optical surfaces with a minimum number of swaps.